As part of the paper-making process, wood chips are disintegrated via the combined actions of alkali white liquor and a digesting machine to yield wood pulp and black liquor. The wood pulp, after being washed to remove the black liquor, is used to produce paper. The now diluted black liquor (containing about 12 to 15% solids) is processed through a multiple-effect evaporator to increase the solids level of the black liquor (to about 45 to 50%). This evaporation of the black liquor results in the generation of foul condensates. These foul condensates are subjected to a steam-stripping process which generates off-gases containing various contaminants. The disposal of such condensate stripper off-gases presents a major problem for the paper industry.
Off-gases contain methanol (CH.sub.3 OH) as well as total reduced sulfur (TRS) gases such as hydrogen sulfide (H.sub.2 S), methyl mercaptan (CH.sub.4 S), dimethyl sulfide [(CH.sub.3).sub.2 S], and dimethyl disulfide [(CH.sub.3).sub.2 S.sub.2]. Unlike dimethyl sulfide and dimethyl disulfide, hydrogen sulfide and methyl mercaptan are weak acids that readily ionize in alkaline solutions to form nonvolatile species. As hydrogen sulfide and methyl mercaptan are classified as being hazardous compounds, their emissions must be reported to the under the Comprehensive Environmental Response and Compensation Liability Act (CERCLA). Several states have passed similar regulations requiring mills to both limit and report emissions of these compounds.
Recent federal regulations have also impacted how paper mills handle the methanol contained in condensate stripper off-gases. It was standard practice in the industry to produce liquid condensate mixtures of methanol and water (from the steam-stripping of the foul condensates) at concentration levels of about 50:50 wt. %. However, hazardous waste regulations now classify such liquids (i.e., liquids having a flash point of less than 140.degree. F.) as being a hazard. Thus, any paper mill which generates a liquid mixture of methanol and water having a flash point of less than 140.degree. F. becomes a hazardous waste generator under federal law.
To avoid becoming a hazardous waste generator at least one paper mill produces liquid condensates from the steam-stripping process having flash points greater than 140.degree. F. This is accomplished by increasing the amount of water contained in the liquid condensate to a level of about 70 wt % or more. As the resulting methanol levels contained in these liquid condensate mixtures are too low to be economically incinerated, the mixtures are sewered. Disposal of methanol in this manner adds a significant biological oxygen demand (BOD) load on the mill's waste treatment plants.
For environmental reasons, it is standard industry practice to incinerate the off-gases generated by the steam-stripping of the foul condensates. Historically such incinerations have been performed in one of three ways. The first method is to directly feed the condensate stripper off-gas stream into a lime kiln for incineration prior to release into the atmosphere. This method results in the removal of about 95% of the sulfur contained in the TRS gases. However, about 5% of the sulfur is released into the atmosphere as sulfur dioxide (SO.sub.2). Thus, direct incineration may not be environmentally feasible for areas under severe sulfur dioxide emission restrictions. An additional problem inherent with this method is ring formation in the lime kilns caused by burning these TRS gases.
The second method used by industry to incinerate the off-gas stream is a variation of the method noted above. The stream is again directly fed into a lime kiln, but the kiln exhaust is sent into a scrubber where caustic is used to remove the sulfur dioxide prior to release into the atmosphere. Although this method greatly reduces the problem of atmospheric sulfur dioxide venting, it does not address the problem of lime kiln ring formation. Furthermore, the system is relatively expensive to implement and operate.
The third method used by industry is to direct the off-gas stream into a dedicated incinerator. After incineration, the stream is sent through a scrubber where caustic is used to remove the SO.sub.2 before release into the atmosphere. While this method both avoids the ring formation problem and greatly reduces the atmospheric release of SO.sub.2, it is also the most expensive of the three methods--requiring capital outlays for an dedicated incinerator and a scrubber as well as their subsequent operating costs.
Therefore, it is the object of this invention to provide an improved economical process for disposal of condensate stripper off-gases. Other objects, features, and advantages will be evident from the following disclosure.